KOSTERLITZ-THOULESS TRANSITION IN THE 2-DIMENSIONAL QUANTUM XY MODEL - COMMENT

We assert that there is insufficient evidence to support a recent claim that the two-dimensional quantum XY model undergoes a Kosterlitz-Thouless phase transition. Contrary to the suggestion of Ding and Makivic, we argue that the real-space renormalization group confirms, like other methods, the existence of some kind of phase transition in such a model and fails, again like other methods, to define the nature of this transition.